Nested positive lookaheads consume characters

[Thom1729]

@deathaxe noticed a bug with nested lookaheads in sublimehq/Packages#2044. I think I've nailed down the problem.

Description

When the last character examined by the expression is inside multiple nested lookaheads, the engine only “unwinds” the innermost lookahead before marking the end of the match.

The bug occurs for rules with push, but not for rules that don't use the stack. (I haven't tested set, pop, or embed.)

Steps to reproduce

Use the following syntax definition:

%YAML 1.2
---
name: Nested lookahead test
scope: source.nested-lookahead-test
contexts:
  main:
    - match: (?=A)
      scope: region.greenish
      push: pop-at-eol
    - match: (?=B(?=1))
      scope: region.greenish
      push: pop-at-eol
    - match: (?=C(?=1(?=2)))
      scope: region.greenish
      push: pop-at-eol
    - match: (?=D(?=1(?=2(?=3))))
      scope: region.greenish
      push: pop-at-eol
    - match: (?=E(?=1(?=2(?=3))))\b
      scope: region.greenish
      push: pop-at-eol
    - match: (?=F(?=1(?=2(?=3))))(?=F)
      scope: region.greenish
      push: pop-at-eol
    - match: (?=G(?=1(?=2(?=3))))(?=G(?=1))
      scope: region.greenish
      push: pop-at-eol

  pop-at-eol:
    - match: $
      pop: true

And the following test file:

A
B1
C12
D123
E123
F123
G123

Expected behavior

No characters in the test file should be green because every rule matches zero characters.

Actual behavior

Many characters are green.

Environment

• Build: 3208
• Mac OS 10.14.6

[wbond]

This was fixed in build 4075